This invention relates to novel linear, phosphine-borane polymers, particularly high molecular weight phosphinoborane polymers; methods for the preparation thereof and, particularly, thermally-induced dehydrocoupling methods.
Carbon comprises less than 0.1% of the Earth""s crust, oceans, and atmosphere. Despite this fact, virtually all polymer systems known and commercially available are based on extended catenated structures containing mainly carbon atoms together with a few other elements, such as oxygen and nitrogen. The availability of suitable organic monomers and the extensive synthetic knowledge associated with organic chemistry allows the design and synthesis of new materials and the subsequent fine-tuning of their properties. In contrast, the development of extended structures based on atoms of other elements has been much less successful and still represents a substantial unsolved synthetic challenge. Nonetheless, the relatively few polymer systems based on inorganic elements such as poly(siloxanes), silicones [R2Sixe2x80x94O]n, polyphosphazenes [R2Pxe2x95x90N]n, polysilanes [R2Si]n and more recently poly(silynes), poly(stannanes), sulfur-containing polymers, poly(metallocenes), and other metallopolymers illustrate the potential for accessing materials with unexpected properties as well as novel applications.
Thermally-induced dehydrocoupling of phosphine-borane adducts R2PH.BH3 at elevated temperatures of, for example, 150-200xc2x0 C. has been previously used to prepare cyclic phosphinoborane species, mainly six-membered rings [R2Pxe2x80x94BH2]3 and, for example, [R2Pxe2x80x94BH2]3 having considerable thermal and hydrolytic stability.(1) In addition in a few cases, low yields of xe2x80x9cpolymericxe2x80x9d materials have been made, although none has been structurally characterized and, where reported, the molecular weights were relatively very low.(2) During the early pioneering work in the 1950""s and 1960""s on boron-phosphorus compounds, the low yield formation of a range of partially characterized, low molecular weight phosphinoborane polymers were described in patents, technical reports and in the academic literature. For example, pyrolysis of Me2Pxe2x80x94PMe2.BH3 or RMePH.BH3 (R=Me or Et) at 175-200xc2x0 C. in the presence of amines, which were claimed to promote the formation of linear rather than cyclic products, was reported to give polymers [RMePxe2x80x94BH2]n with molecular weights of 1,800-6,000 (where determined).(2,3) For a general survey of results obtained during this period see G. W. Parshall in xe2x80x9cThe Chemistry of Boron and its Compoundsxe2x80x9d: E. L. Muetterties Ed., Wiley, N.Y. (1967) Ch. 9 p 617-646. Dehydrocoupling routes to bonds between inorganic elements have provided important routes to Group 14 polymers.(4) 
Dehydrocoupling has been used to form oligomers and polymers with Bxe2x80x94N bonds between borazine rings,(5) while coordinate bonds between B and N have recently been used in the preparation of metallopolymers.(6) 
The phosphine-borane adduct Ph2PH.BH3 is known to undergo dehydrocoupling at 180-190xc2x0 C. and above over a period of 14 h to exclusively and quantitatively yield the cyclic trimer [Ph2Pxe2x80x94BH2]3.(7) 
The preparation of very low molecular weight polymers of Mn 1480-2630 from the thermolysis of PhPH2.BH3 at 150-250xc2x0 C. in the absence of a catalyst has been described.(8) 
1. C. A. B. Burg and R. I. Wagner, J. Am. Chem. Soc. (1953) 75. 3872.
2. R. I Wagner and F. F. Caserio, J. Inorg. Nucl. Chem. (1959), 11, 259.
3. A. B. Burg, J. Inorg. Nucl. Chem. (1959), 11, 258.
4. See, for example, (a) P. Bianconi, T. W. Weidman J. Am. Chem. Soc. (1988), 22, 1697. (b) T. Imori, T. D. Tilley J. Chem. Soc. Chem. Commun. (1993), 1607. (c) I. Manners, G. Renner, H. R. Allcock, O. Nuyken J. Am. Chem. Soc., (1989), 111, 5478. (d) J. A. Dodge, I. Manners, G. Renner, H. R. Allcock, O. Nuyken, J. Am. Chem. Soc (1990), 112, 1268. (e) M. Liang, I. Manners, J. Am. Chem. Soc., (1991), 113, 4044. (f) A. K. Roy J. Am. Chem. Soc. (1992), 114, (g) V. Chunechom, T. E. Vidal, H. Adams, M. L. Turner Angew. Chem. Int. Ed. Engl. (1998), 37, 1928.
5. P. J. Fuzan et al. Chem. Mater. (1990), 2, 96.
6. M. Fontani et al. Eur. J. Inorg. Chem. (1998), 2087.
7. W. Gee et al. J. Chem. Soc. (1965), 3171.
8. V. V. Korshak et al. Izv. Akad. Nauk SSR, Ser. Khim, (1964), 1541.
The present invention provides novel, polymeric compounds having a linear backbone of alternating phosphorus and boron atoms.
The invention further provides novel, optionally, metal catalysed dehydrocoupling methods to produce linear phosphorus-boron polymers.
Accordingly, in one aspect the invention provides linear backbone phosphorus-boron polymers of the general formula (I) 
wherein R1-R4 are the same or different and selected from H, optionally substituted lower alkyl, alkenyl and aryl; and n is at least 2. Preferably, the invention provides a polyphenylphosphinoborane of aforesaid formula (I) wherein R1, H and R4 is phenyl. More preferably, the linear polymers as hereinabove defined included low molecular weight oligomers having weight average molecular weights (Mw) of about 5,000 and higher molecular weight polymers having Mw more preferably of at least 10,000, and still more preferably at least 20,000.
In a further aspect, the invention provides a method for producing polymers having a linear backbone of alternating phosphorus and boron atoms, said method comprising dehydrocoupling a phosphine-boron adduct by treating said adduct at a temperature to effect said dehydrocoupling to produce said linear polymer.
Preferably, the aforesaid process is carried out at effective temperatures lower than a temperature which would produce a corresponding phosphorus-boron cyclic trimer compound.
More preferably, the dehydrocoupling methods as hereinabove defined further include the presence of an effective dehydrocoupling catalyst, for example, complexed Rh(I) catalysts or complexes of other metals.
Specific Examples are:
[Rh(1,5-cod)2][OTf]
[Rh(PPh3)3Cl]
[Rh(1,5-cod)(dmpe)][PF6]
[Rh(CO)(PPh3)3H]
anhydrous RhCl3 
RhCl3 hydrate
[{Cp*Rh(xcexcCl)Cl}2]
[{Ir(xcexc-Cl)Cl}2]
[Ir(1,5-cod)2][BF4]
Cp2TiMe2 
Ru3(CO)12 
[Pt(1,5-cod)2]
PdCl2 
PtCl2; and most preferably
[{Rh(xcexc-Cl)(1,5-cod)}2]
In the foregoing list of compounds, it will be understood that cod represents cyclooctadiene; OTf stands for triflate anion (CF3SO2Oxe2x88x92); and Dmpe is dimethylphosphinoethane.
In a further aspect, the invention provides a method of producing the dimer compound of formula I, wherein R1 and R2 are Ph, R3 and R4 are H and n is 2, i.e. 
by treating the phosphine-borane adduct Ph2PHxe2x80x94BH3 at a dehydrocoupling temperature of less than 180xc2x0 C., preferably in the presence of an effective amount of a dehydrocoupling catalyst.
In alternative methods for producing high molecular weight polymers according to the invention, alkylated derivatives may be made by alkylation of labile hydrogen-bearing phosphorus in the polymers by, for example, reaction with alkylated lithium or Grignard reagents, viz: 
Wherein R2 is an optionally substituted lower alkyl or aryl.